Anyone notice if their speedometer is off by a few MPH?

[ScottsGT]

Driving in to work, I've come across the portable roadside speed limit signs that tell you how fast you are going and start flashing if you are really going too fast. My first encounter I noticed a different reading, but it just didn't register with me.

This morning when I came across one again, I had a thought that went off like a light bulb in my head. I noticed that my speedometer was reading 40 MPH, but the sign was telling me I was doing 36-37 MPH.
If this is truly off, what about odometer readings? I know for a fact, since I owned one, Honda had a notice go out to all owners of Odysseys back in '04 that their odometers were off and you had to subtract 7% from the reading to get the actual mileage. This had a big effect on warranties and resale values. We had the 100,000 mile extended warranty, so it actually was good up to 107,000 miles.

On my way home tonight, I'm going to use my GPS speed app and double check my speeds against the cars speedometer and I'll try some mile markers against my odometer. If it truly is off again, how can I go about verifying actual mileage and speed readings?

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[GPhilly04]

Driving in to work, I've come across the portable roadside speed limit signs that tell you how fast you are going and start flashing if you are really going too fast. My first encounter I noticed a different reading, but it just didn't register with me.

This morning when I came across one again, I had a thought that went off like a light bulb in my head. I noticed that my speedometer was reading 40 MPH, but the sign was telling me I was doing 36-37 MPH.
If this is truly off, what about odometer readings? I know for a fact, since I owned one, Honda had a notice go out to all owners of Odysseys back in '04 that their odometers were off and you had to subtract 7% from the reading to get the actual mileage. This had a big effect on warranties and resale values. We had the 100,000 mile extended warranty, so it actually was good up to 107,000 miles.

On my way home tonight, I'm going to use my GPS speed app and double check my speeds against the cars speedometer and I'll try some mile markers against my odometer. If it truly is off again, how can I go about verifying actual mileage and speed readings?

Kiiiiiiinda related thread here. Maybe something in there will be useful.

Some people (myself included) speculate that it's just the angle of the speedo from the center of the drivers seat. Because it's offset a few inches to the right, the speedo makes you think you're going a few MPH faster than you really are.

http://www.mustang6g.com/forums/showthread.php?t=19766

 

[GrabberBlue5.0]

Yea mine does this as well, I'm not sure how far off it is but even my cruise control reads about 1-2 mph low compared to my speedo at highway speeds. Not really sure why


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[Horde5.0]

From the other thread

"Most car's speedo in america shows slightly higher, to help deter you in speeding. So I believe the cruise control wasn't set to compensate for that, so your setting the cruise control to the actual speed but the speedo is still showing it's slightly higher calibration."

This is apparently true. My Volkswagen was like this. There was a way to get the actual speed to display in the climate control window so I could see the difference.

 

[P4RKER]

My cruise control and speedo show identical speeds but my radar detector and phone GPS show my speed as 2mph faster than my car reads. Makes me nervous because in both my other cars the phone GPS reads exactly with my speedo. I would hate to think I'm only 9 over and pass a cop and be 11 over and get a ticket for a wacky speedo.

 

[Kevin08]

So if someone straps a bomb to the bottom of your car and tells you to maintain 55mph or explode, You might be fucked if you're not going at least 58?

You poor bastards

 

[wildsailor]

Nope, nailed it with the alignment issue...from the driver seat the speedo (er, Ground Speed) pod is angled towards the center line of the car. Move your head to the right and you will see the speed align now with where it is supposed to be. The Fuel Gauge has the same issue!

 

[fionic]

keep in mind those signs aren't always accurate either.

also, wheel/tire size affects it as well.

 

[fionic]

My cruise control and speedo show identical speeds but my radar detector and phone GPS show my speed as 2mph faster than my car reads. Makes me nervous because in both my other cars the phone GPS reads exactly with my speedo. I would hate to think I'm only 9 over and pass a cop and be 11 over and get a ticket for a wacky speedo.

ask them to see the radar gun, and also the last time it was calibrated.

radar isn't perfect.

 

[sstevens7]

noticed this as well. the actual speedometer is off a few but when I put it in cruise and go past the radar gun the indicated speed on the cruise is the same displayed on the radar sign. noticed my speedometer is off 1 0r 2 from what is displayed on the cruise speed.

 

[ScottsGT]

So if someone straps a bomb to the bottom of your car and tells you to maintain 55mph or explode, You might be fucked if you're not going at least 58?

You poor bastards

SON OF A BITCH!! :eyebulge: I better get this fixed and quick!! :eyebulge:

 

[Charles147]

All speedometers are slightly off. Doesn't matter what car you drive.

 

[Sasuketr]

For gtpp owners, the car's actual speed is 1-2 mph faster than what the speedo reads. I confirmed this with my bud's gps. This also affects the track apps as well. Sadly but true.

 

[10splaya22]

You can verify it with a GPS as I wouldn't trust that sign 100%. If you have access to a radar gun that would be even better.

How accurate are speedometers?
Dan Edmunds, an automotive engineer and the director of vehicle testing at Edmunds.com, says that speedometers cannot have an error of more than 5 percent (typically expressed as plus/minus 2.5 percent relative to the actual speed) according to federal law.

"At 60 mph this means your speedometer should read no more than 1.5 mph higher or lower than your actual speed," Edmunds explains.

Manufacturing tolerances and difference between tire brands and air pressure can cause tire diameters to vary, which may lead to an erroneous speedometer. Automakers offer different sized wheels and tires, and, while carmakers try to match the effective rolling diameter of the larger wheels with a lower profile tire, an exact match is not always possible.

"The tires size steps are small enough that the diameter can easily be kept within a percent or two, which allows the same gearing and speedometer calibration factors to be carried over because everything still fits within the speedometer tolerance," Edmunds says.

However, Edmunds adds that any change in the effective rolling diameter caused by replacement tires will bring with it speedometer error.

http://www.foxnews.com/leisure/2012/05/11/how-fast-are-really-going-accuracy-speedometers/

 

[Sasuketr]

So if someone straps a bomb to the bottom of your car and tells you to maintain 55mph or explode, You might be fucked if you're not going at least 58?

You poor bastards

Was that from the movie Speed with Keanu Reeves?

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